Microfiltration and ultrafiltration membrane filters are increasingly used in municipal drinking water treatment and wastewater treatment. With a typical microfiltration or ultrafiltration operation, transmembrane pressure (TMP) is the main driving force for the filtration process. As the process progresses and filtered water permeates through the membrane, most of the solid contaminants are retained either on the membrane surface, forming a fouling layer or as plugs in the membrane pores blocking filtration. The presence of a fouling layer or plugged pores or both can significantly increase the transmembrane pressure and ultimately the filtration energy consumption.
Frequent backwashes with either filtrate or gas can partially reclaim the membrane permeability, however, with these methods it is known that foulants may continue to build up on the membrane surface and in the pores. Successive backwashes do not remove all the deposited material, and consequently over time there is deterioration in filter permeability.
Cleaning protocols supplemented by the use of chemical cleaning agents are known. The general criteria for selecting a chemical cleaning method are firstly to achieve a good recovery of membrane permeability, secondly to minimise the amount of chemical cleaning agent required and the resultant amount of waste generated and thirdly to allow ease of operation.
One cleaning method involves immersing the membranes in a chemical cleaning solution for a period of time. The chemical reactions dissolve the solids accumulated on the membrane surface and plugged in the pores. The cleaning effect may be facilitated by injecting air to scour membranes or by recirculating the chemical solution. This is a common method currently used for full chemical recovery of membrane performance, however it suffers from the drawback that a significant volume of chemical solution is required and the process generates large volumes of chemical waste.
In other attempts to control permeability deterioration, short chemical cleans (maintenance cleans) are frequently carried out with membrane filters. U.S. Pat. No. 5,403,479 describes one such method of carrying out such a process. A cleaning solution is introduced into the lumen of a hollow fibre membrane, and recirculated at low flow at a pressure below the membrane bubble point. The cleaning solution permeates through membrane pores under low transmembrane pressure and at a low rate. The authors also indicate that diffusion of cleaning solution through membrane may occur even when fluid is held in the fibres at no velocity but under pressure. During the course of cleaning, the membrane is immersed in water. This method requires storage of a volume of chemical solution sufficient to fill the permeate lines and for recirculation. During the recirculation process, an increasing amount of chemical solution penetrates through the membrane pores from the lumen side to the feed side.
Another method of conducting a membrane clean involves repeatedly backpulsing membranes with a chemical cleaner, as described in JP09313902 and U.S. Pat. No. 6,045,698. Similar methods of chemical cleaning backwash methods for submerged membrane systems are disclosed in US Patent applications 20010052494A1, 20030146153A1, 20040007525A1, PCT application WO0108790A1 and U.S. Pat. No. 6,547,968. Chemical cleaning backwash can be continuous or pulsed, and can be carried out after the membrane tank is drained, while the membrane tank is draining, or even without draining the membrane tank prior to the chemical backwashes. The chemical solution is delivered to the system via a header at the top of the membranes by pump when the membranes are oriented vertically. The chemical cleaning solution is forced under pressure through the membranes to the lumen side. Repeated backpulses may bring the biofilm in the permeate lines back to membranes.
The second and third methods mentioned above generally use less volume of chemical solution than the first method, but they require that the chemical solution be of very high quality and free of solids because the solution is introduced into the permeate side.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.